1. Field of the Invention
This invention relates to a flat cathode ray display tube comprising an envelope including a substantially flat, transparent, faceplate carrying a phosphor screen, means for producing an electron beam and directing the beam parallel to the faceplate through a first region towards a reversing lens which turns the beam so that it travels in the opposite direction parallel to the faceplate through a second region, first deflection means intermediate the electron beam producing means and the reversing lens for deflecting the beam in a plane substantially parallel to the faceplate to effect line scanning, and second deflection means in the second region for deflecting the electron beam toward the screen, and operable to effect field scanning.
2. Description of the Related Art
A flat cathode ray display tube of this kind is described in British Patent Specification No. 2101396. In this tube, the envelope consists of a shallow, generally rectangular, metal can with a flat glass faceplate constituting the display window mounted on the can. An electron gun in the rear region of the envelope produces a low energy electron beam which is deflected linewise by an adjacent electrostatic deflection arrangement before passing to the reversing lens. After having been reversed through 180.degree., the beam undergoes field scanning by means of a plurality of selectively energised, vertically spaced, horizontally elongate electrodes arranged in a plane parallel with the faceplate in the front region of the envelope and is deflected thereby towards a phosphor screen carried on the faceplate onto the input side of a channel electron multiplier disposed parallel to, but spaced from, the screen. Thus, the line and field scanned beam provides a raster scanning electron input to the electron multiplier. Having undergone current multiplication within the electron multiplier, the electron beam is accelerated onto the phosphor screen by means of a high voltage field established between a backing electrode on the screen and the output side of the electron multiplier to produce a raster-scanned display picture.
An advantage in using an electron multiplier in this manner is that the multiplier in effect separates the scanning function of the electron beam from the light-generating process. The electron beam, prior to reaching the multiplier, need only be of low energy so that the beam forming and raster scanning section of the tube operates at low voltage and current compared with the high voltage, higher current screen output section. The term "low energy" used herein is intended to signify an electron beam of less than 2.5 KeV and typically several hundred electron volts. For example, a low voltage, low current beam having an acceleration voltage of around 600 V may be used. The electron multiplier amplifies the beam current with the amplified current, on leaving the multiplier, being accelerated across a short gap to the screen to produce the power necessary to generate the light output. The low energy and low current electron beam used in the beam forming and raster scanning section of the tube can easily be deflected through large angles without undue enlargement of the spot. This enables the kind of folded electron optical system described to be employed with the result that a comparatively compact, and shallow, display tube is obtained.
However, it has been found that, as a result of the use of a low energy electron beam in the beam forming and raster scanning section of the tube and the long trajectory of the beam in that section, the tube is more sensitive to ambient magnetic fields, for example the Earth's magnetic field, than a conventional display tube using a high voltage beam. Ambient magnetic fields penetrating this section of the tube can, for example, influence the direction and position of the beam before it reaches the reversing lens producing a deviation from the intended path of the beam through the reversing lens.
The metal can of the tube's envelope affords some magnetic shielding. In addition, an external magnetic shield comprising a box of mumetal material can be fitted around the tube's envelope, but not, of course covering the faceplate. Where Hx, Hy, and Hz designate magnetic field components along three mutually perpendicular axes, x, y, and z, extending respectively parallel to the line deflection direction (i.e. perpendicular to the output beam of the gun and parallel to the faceplate), parallel to the axis of the electron gun (i.e. parallel to the output beam of the gun and the faceplate), and perpendicular to the plane of the faceplate such an external magnetic shield can reduce the susceptibility to the Hx and Hy components to a sufficient level to allow operation of the tube at any orientation with respect to, for example, the Earth's magnetic field without serious effect.
The practical limit of this shielding is determined by the leakage of the external magnetic field through the faceplate. This leakage is greatest for the Hz component. The Hz component entering through the faceplate can result in a shift of the raster along the x direction, parallel to line scan direction. In a tube having a screen of approximately 120 mm (field height) by 160 mm (line width), a maximum electron beam trajectory of approximately 350 mm and a beam voltage of 600 V, this shift in the x direction may be, for an Hz component measured in Amperes/meter around 0.13 to 0.19 mm per Ampere/meter. Whilst this shift in the position of the raster on the screen can be tolerated and electronically corrected for static applications, it becomes more significant when the tube is used in a mobile environment.